The present invention relates to a method for inspecting a crack in a metal surface or the like, and particularly, to an inspection method and apparatus for nondestructive inspection such as liquid penetrant inspection and magnetic particle testing.
Liquid penetrant inspection and magnetic particle testing are nondestructive inspection methods for inspecting a flaw such as a fracture (crack) in a metal surface. In the liquid penetrant inspection, a red liquid (“penetrant”) is typically applied to an inspected surface. The penetrant is wiped off after a predetermined period, and a white powder (“developer”) is applied to the surface. If there is a flaw such as a crack, the developer draws the penetrant remaining in the crack to the surface, revealing the location, shape and size of the flaw.
In magnetic particle testing, a solution that includes fluorescent magnetic powder is applied to a magnetic object under inspection; then the object is magnetized. If there is a flaw such as a crack, the magnetic flux concentrates on the flaw, attracting the fluorescent magnetic powder. Under ultraviolet light, the magnetic powder emits green light which outlines any flaw. Conventionally, verification of these flaw indicators has been only by visual observation.
In the case of conventional inspection by visual observation, a flaw may be overlooked because of an inspector's fatigue, or the inspection results may differ due to differences of perception or opinion among individual inspectors. Further, the inspection result is recorded merely as a word, “passed” or the like. Thus, there is a problem in reliability of inspection.
Moreover, in magnetic particle testing, an automatic inspection apparatus has been developed for use with important and mass-produced parts; however, the apparatus is specialized, and various shaped parts cannot be readily inspected.
Further, in liquid penetrant inspection surface colors must be detected as a highly precise two-dimensional distribution. Even if a colorimeter capable of high precision chromaticity measurement is used, two-dimensional sweeping is necessary. Thus, it is difficult from the standpoint of inspection time and cost to automatically inspect various shaped parts.
In addition, when an object under inspection is large, it may be difficult to distinguish the part of the object corresponding to an image obtained by automatic inspection, or to discern the part of the object corresponding to a detected flaw.
Further, the economical value will be dramatically improved if both of the liquid penetrant inspection and the magnetic particle testing could be performed automatically by one apparatus.